Percentage calculator
The four percentage questions people actually ask, answered as you type: a percentage of a number, one number as a percentage of another, percentage change, and adding or removing a percentage.
Pick the question you have
Every box is live — type and the answer updates.
1. What is X% of Y?
2. X is what percent of Y?
3. Percentage change from X to Y
4. Add or subtract X% from Y
The four formulas, written out
- Percent of a number:
result = number × percent ÷ 100. 17% of 1,200 is 1,200 × 0.17 = 204. - One number as a percent of another:
percent = part ÷ whole × 100. 45 of 200 is 22.5%. - Percentage change:
change % = (new − old) ÷ old × 100. From 400 to 500 is +25%; from 400 to 300 is −25%. - Add or subtract a percent:
result = amount × (1 ± percent ÷ 100). 1,000 + 17% = 1,170; 1,000 − 17% = 830.
Percentages do not cancel
A 20% fall followed by a 20% rise does not return you to where you started. 100 → 80 → 96. The fall is measured from 100, the rise from 80, so the second step is smaller in absolute terms. To recover a 20% fall you need a 25% rise.
The same trap appears in reverse: a price with tax already included must be divided by 1 + rate, not multiplied by 1 − rate. On 1,170 including 17% GST, the tax is 170 and the base is 1,000.
Percent is also the shared language of the money tools here: Zakat is 2.5%, an invoice can carry a tax rate and a discount, and a loan is quoted as an annual percentage rate.
Questions people ask
What does “percent” actually mean?
Percent means “per hundred”: 17% is 17 out of every 100, the same as the decimal 0.17. To take a percentage of a number, multiply it by the percentage divided by 100.
How do I calculate percentage increase?
Subtract the old value from the new value, divide by the old value, then multiply by 100. Going from 400 to 500 is (500 − 400) ÷ 400 × 100 = 25%. A fall uses the same formula and gives a negative result.
Why is a 20% discount followed by 20% tax not a wash?
Because the second percentage is taken from a different base. On 1,000, a 20% discount leaves 800, and 20% tax on 800 is 160 — a final price of 960, not 1,000.
How do I add GST or VAT to a price?
Multiply by one plus the rate: 1,200 at 17% GST is 1,200 × 1.17 = 1,404. To remove tax that is already included, divide by one plus the rate: 1,404 ÷ 1.17 = 1,200.
Is anything I type stored or sent?
No. The maths runs in this page, in this tab. There is no upload and no account.